TWISTED DWARF1 Mediates the Action of Auxin Transport Inhibitors on Actin Cytoskeleton Dynamics

ABCB transporters: functionality extends to more than auxin transportation

MAIN CONCLUSION

ABCs transport diverse compounds; with plant's most abundant ABCG and ABCB subfamilies. ABCBs are multi-functional transporter proteins having role in plant adaptation. ATP-binding cassette (ABC) proteins have been known for the transportation of various structurally diverse compounds in all kingdoms of life. Plants possess a particularly high number of ABC transporters compared to other eukaryotes: the most abundant being ABCG followed by the ABCB subfamilies. While members of the ABCB subfamily are primarily known for auxin transportation, however, studies have shown their involvement in variety of other functions viz. growth and development, biotic and abiotic stresses, metal toxicity and homeostasis, cellular redox state stability, stomatal regulation, cell shape maintenance, and transport of secondary metabolites and phytohormones. These proteins are able to perform various biological processes due to their widespread localization in the plasma membrane, mitochondrial membrane, chloroplast, and tonoplast facilitating membrane transport influenced by various environmental and biological cues. The current review compiles published insights into the role of ABCB transporters, and also provides brief insights into the role of ABCB transporters in a medicinal plant, where the synthesis of its bioactive secondary metabolite is linked to the primary function of ABCBs, i.e., auxin transport. The review discusses ABCB subfamily members as multi-functional protein and comprehensively examines their role in various biological processes that help plants to survive under unfavorable environmental conditions.

Actin-mediated avoidance of tricellular junction influences global topology at the Arabidopsis shoot apical meristem

Division plane orientation contributes to cell shape and topological organization, playing a key role in morphogenesis, but the precise physical and molecular mechanism influencing these processes remains largely obscure in plants. In particular, it is less clear how the placement of the new walls occurs in relation to the walls of neighboring cells. Here, we show that genetic perturbation of the actin cytoskeleton results in more rectangular cell shapes and higher incidences of four-way junctions, perturbing the global topology of cells in the shoot apical meristem of Arabidopsis thaliana. Actin mutants also exhibit changes in the expansion rate of the new versus the maternal cell wall after division, affecting the evolution of internal angles at tricellular junctions. Further, the increased width of the preprophase band in the actin mutant contributes to inaccuracy in the placement of the new cell wall. Computational simulation further substantiates this hypothesis and reproduces the observed cell shape defects.

Structure and function of the Arabidopsis ABC transporter ABCB19 in brassinosteroid export

Brassinosteroids are steroidal phytohormones that regulate plant development and physiology, including adaptation to environmental stresses. Brassinosteroids are synthesized in the cell interior but bind receptors at the cell surface, necessitating a yet to be identified export mechanism. Here, we show that a member of the ATP-binding cassette (ABC) transporter superfamily, ABCB19, functions as a brassinosteroid exporter. We present its structure in both the substrate-unbound and the brassinosteroid-bound states. Bioactive brassinosteroids are potent activators of ABCB19 ATP hydrolysis activity, and transport assays showed that ABCB19 transports brassinosteroids. In Arabidopsis thaliana, ABCB19 and its close homolog, ABCB1, positively regulate brassinosteroid responses. Our results uncover an elusive export mechanism for bioactive brassinosteroids that is tightly coordinated with brassinosteroid signaling.

A multi-trait GWAS-based genetic association network controlling soybean architecture and seed traits

Ideal plant architecture is essential for enhancing crop yields. Ideal soybean (Glycine max) architecture encompasses an appropriate plant height, increased node number, moderate seed weight, and compact architecture with smaller branch angles for growth under high-density planting. However, the functional genes regulating plant architecture are far not fully understood in soybean. In this study, we investigated the genetic basis of 12 agronomic traits in a panel of 496 soybean accessions with a wide geographical distribution in China. Analysis of phenotypic changes in 148 historical elite soybean varieties indicated that seed-related traits have mainly been improved over the past 60 years, with targeting plant architecture traits having the potential to further improve yields in future soybean breeding programs. In a genome-wide association study (GWAS) of 12 traits, we detected 169 significantly associated loci, of which 61 overlapped with previously reported loci and 108 new loci. By integrating the GWAS loci for different traits, we constructed a genetic association network and identified 90 loci that were associated with a single trait and 79 loci with pleiotropic effects. Of these 79 loci, 7 hub-nodes were strongly linked to at least three related agronomic traits. qHub_5, containing the previously characterized Determinate 1 (Dt1) locus, was associated not only with plant height and node number (as determined previously), but also with internode length and pod range. Furthermore, we identified qHub_7, which controls three branch angle-related traits; the candidate genes in this locus may be beneficial for breeding soybean with compact architecture. These findings provide insights into the genetic relationships among 12 important agronomic traits in soybean. In addition, these studies uncover valuable loci for further functional gene studies and will facilitate molecular design breeding of soybean architecture.

A Glu209Lys substitution in DRG1/TaACT7, which disturbs F-actin organization, reduces plant height and grain length in bread wheat

Plant height and grain size are two important agronomic traits that are closely related to crop yield. Numerous dwarf and grain-shape mutants have been studied to identify genes that can be used to increase crop yield and improve breeding programs. In this study, we characterized a dominant mutant, dwarf and round grain 1 (drg1-D), in bread wheat (Triticum aestivum L.). drg1-D plants exhibit multiple phenotypic changes, including dwarfism, round grains, and insensitivity to brassinosteroids (BR). Cell structure observation in drg1-D mutant plants showed that the reduced organ size is due to irregular cell shape. Using map-based cloning and verification in transgenic plants, we found that a Glu209Lys substitution in the DRG1 protein is responsible for the irregular cell size and arrangement in the drg1-D mutant. DRG1/TaACT7 encodes an actin family protein that is essential for polymerization stability and microfilament (MF) formation. In addition, the BR response and vesicular transport were altered by the abnormal actin cytoskeleton in drg1-D mutant plants. Our study demonstrates that DRG1/TaACT7 plays an important role in wheat cell shape determination by modulating actin organization and intracellular material transport, which could in the longer term provide tools to better understand the polymerization of actin and its assembly into filaments and arrays.

Synergism of vesicle trafficking and cytoskeleton during regulation of plant growth and development: A mechanistic outlook

The cytoskeleton is a fundamental component found in all eukaryotic organisms, serving as a critical factor in various essential cyto-biological mechanisms, particularly in the locomotion and morphological transformations of plant cells. The cytoskeleton is comprised of three main components: microtubules (MT), microfilaments (MF), and intermediate filaments (IF). The cytoskeleton plays a crucial role in the process of cell wall formation and remodeling throughout the growth and development of cells. It is a highly organized and regulated network composed of filamentous components. In the basic processes of intracellular transport, such as mitosis, cytokinesis, and cell polarity, the plant cytoskeleton plays a crucial role according to recent studies. The major flaws in the organization of the cytoskeletal framework are at the root of the aberrant organogenesis currently observed in plant mutants. The regulation of protein compartmentalization and abundance within cells is predominantly governed by the process of vesicle/membrane transport, which plays a crucial role in several signaling cascades.The regulation of membrane transport in eukaryotic cells is governed by a diverse array of proteins. Recent developments in genomics have provided new tools to study the evolutionary relationships between membrane proteins in different plant species. It is known that members of the GTPases, COP, SNAREs, Rabs, tethering factors, and PIN families play essential roles in vesicle transport between plant, animal, and microbial species. This Review presents the latest research on the plant cytoskeleton, focusing on recent developments related to the cytoskeleton and summarizing the role of various proteins in vesicle transport. In addition, the report predicts future research direction of plant cytoskeleton and vesicle trafficking, potential research priorities, and provides researchers with specific pointers to further investigate the significant link between cytoskeleton and vesicle trafficking.

TaACTIN7-D regulates plant height and grain shape in bread wheat

Exploitation of new gene resources and genetic networks contributing to the control of crop yield-related traits, such as plant height, grain size, and shape, may enable us to breed modern high-yielding wheat varieties through molecular methods. In this study, via ethylmethanesulfonate mutagenesis, we identify a wheat mutant plant, mu-597, that shows semi-dwarf plant architecture and round grain shape. Through bulked segregant RNA-seq and map-based cloning, the causal gene for the semi-dwarf phenotype of mu-597 is located. We find that a single-base mutation in the coding region of TaACTIN7-D (TaACT7-D), leading to a Gly-to-Ser (G65S) amino acid mutation at the 65th residue of the deduced TaACT7-D protein, can explain the semi-dwarfism and round grain shape of mu-597. Further evidence shows that the G65S mutation in TaACT7-D hinders the polymerization of actin from monomeric (G-actin) to filamentous (F-actin) status while attenuates wheat responses to multiple phytohormones, including brassinosteroids, auxin, and gibberellin. Together, these findings not only define a new semi-dwarfing gene resource that can be potentially used to design plant height and grain shape of bread wheat but also establish a direct link between actin structure modulation and phytohormone signal transduction.

Basally distributed actin array drives embryonic hypocotyl elongation during the seed-to-seedling transition in Arabidopsis

Seed germination is a vital developmental transition for the production of progeny by sexual reproduction in spermatophytes. The seed-to-seedling transition is predominately driven by hypocotyl cell elongation. However, the mechanism that underlies hypocotyl growth remains largely unknown. In this study, we characterized the actin array reorganization in embryonic hypocotyl epidermal cells. Live-cell imaging revealed a basally organized actin array formed during hypocotyl cell elongation. This polarized actin assembly is a barrel-shaped network, which comprises a backbone of longitudinally aligned actin cables and a fine actin cap linking these cables. We provide genetic evidence that the basal actin array formation requires formin-mediated actin polymerization and directional movement of actin filaments powered by myosin XIs. In fh1-1 and xi3ko mutants, actin filaments failed to reorganize into the basal actin array, and the hypocotyl cell elongation was inhibited compared with wild-type plants. Collectively, our work uncovers the molecular mechanisms for basal actin array assembly and demonstrates the connection between actin polarization and hypocotyl elongation during seed-to-seedling transition.

Galactosylation of xyloglucan is essential for the stabilization of the actin cytoskeleton and endomembrane system through the proper assembly of cell walls

Xyloglucan, a major hemicellulose, interacts with cellulose and pectin to assemble primary cell walls in plants. Loss of the xyloglucan galactosyltransferase MURUS3 (MUR3) leads to the deficiency of galactosylated xyloglucan and perturbs plant growth. However, it is unclear whether defects in xyloglucan galactosylation influence the synthesis of other wall polysaccharides, cell wall integrity, cytoskeleton behaviour, and endomembrane homeostasis. Here, we found that in mur3-7 etiolated seedlings cellulose was reduced, CELLULOSE SYNTHASE (CESA) genes were down-regulated, the density and mobility of cellulose synthase complexes (CSCs) were decreased, and cellulose microfibrils become discontinuous. Pectin, rhamnogalacturonan II (RGII), and boron contents were reduced in mur3-7 plants, and B-RGII cross-linking was abnormal. Wall porosity and thickness were significantly increased in mur3-7 seedlings. Endomembrane aggregation was also apparent in the mur3-7 mutant. Furthermore, mutant seedlings and their actin filaments were more sensitive to Latrunculin A (LatA) treatment. However, all defects in mur3-7 mutants were substantially restored by exogenous boric acid application. Our study reveals the importance of MUR3-mediated xyloglucan galactosylation for cell wall structural assembly and homeostasis, which is required for the stabilization of the actin cytoskeleton and the endomembrane system.

Recent advances in auxin biosynthesis and homeostasis

The plant proliferation is linked with auxins which in turn play a pivotal role in the rate of growth. Also, auxin concentrations could provide insights into the age, stress, and events leading to flowering and fruiting in the sessile plant kingdom. The role in rejuvenation and plasticity is now evidenced. Interest in plant auxins spans many decades, information from different plant families for auxin concentrations, transcriptional, and epigenetic evidences for gene regulation is evaluated here, for getting an insight into pattern of auxin biosynthesis. This biosynthesis takes place via an tryptophan-independent and tryptophan-dependent pathway. The independent pathway initiated before the tryptophan (trp) production involves indole as the primary substrate. On the other hand, the trp-dependent IAA pathway passes through the indole pyruvic acid (IPyA), indole-3-acetaldoxime (IAOx), and indole acetamide (IAM) pathways. Investigations on trp-dependent pathways involved mutants, namely yucca (1-11), taa1, nit1, cyp79b and cyp79b2, vt2 and crd, and independent mutants of tryptophan, ins are compiled here. The auxin conjugates of the IAA amide and ester-linked mutant gh3, iar, ilr, ill, iamt1, ugt, and dao are remarkable and could facilitate the assimilation of auxins. Efforts are made herein to provide an up-to-date detailed information about biosynthesis leading to plant sustenance. The vast information about auxin biosynthesis and homeostasis is consolidated in this review with a simplified model of auxin biosynthesis with keys and clues for important missing links since auxins can enable the plants to proliferate and override the environmental influence and needs to be probed for applications in sustainable agriculture.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s13205-023-03709-6.

Kaempferol-3-rhamnoside overaccumulation in flavonoid 3'-hydroxylase tt7 mutants compromises seed coat outer integument differentiation and seed longevity

Seeds slowly accumulate damage during storage, which ultimately results in germination failure. The seed coat protects the embryo from the external environment, and its composition is critical for seed longevity. Flavonols accumulate in the outer integument. The link between flavonol composition and outer integument development has not been explored. Genetic, molecular and ultrastructural assays on loss-of-function mutants of the flavonoid biosynthesis pathway were used to study the effect of altered flavonoid composition on seed coat development and seed longevity. Controlled deterioration assays indicate that loss of function of the flavonoid 3' hydroxylase gene TT7 dramatically affects seed longevity and seed coat development. Outer integument differentiation is compromised from 9 d after pollination in tt7 developing seeds, resulting in a defective suberin layer and incomplete degradation of seed coat starch. These distinctive phenotypes are not shared by other mutants showing abnormal flavonoid composition. Genetic analysis indicates that overaccumulation of kaempferol-3-rhamnoside is mainly responsible for the observed phenotypes. Expression profiling suggests that multiple cellular processes are altered in the tt7 mutant. Overaccumulation of kaempferol-3-rhamnoside in the seed coat compromises normal seed coat development. This observation positions TRANSPARENT TESTA 7 and the UGT78D1 glycosyltransferase, catalysing flavonol 3-O-rhamnosylation, as essential players in the modulation of seed longevity.

An LRR receptor kinase controls ABC transporter substrate preferences during plant growth-defense decisions

The exporter of the auxin precursor indole-3-butyric acid (IBA), ABCG36/PDR8/PEN3, from the model plant Arabidopsis has recently been proposed to also function in the transport of the phytoalexin camalexin. Based on these bonafide substrates, it has been suggested that ABCG36 functions at the interface between growth and defense. Here, we provide evidence that ABCG36 catalyzes the direct, ATP-dependent export of camalexin across the plasma membrane. We identify the leucine-rich repeat receptor kinase, QIAN SHOU KINASE1 (QSK1), as a functional kinase that physically interacts with and phosphorylates ABCG36. Phosphorylation of ABCG36 by QSK1 unilaterally represses IBA export, allowing camalexin export by ABCG36 conferring pathogen resistance. As a consequence, phospho-dead mutants of ABCG36, as well as qsk1 and abcg36 alleles, are hypersensitive to infection with the root pathogen Fusarium oxysporum, caused by elevated fungal progression. Our findings indicate a direct regulatory circuit between a receptor kinase and an ABC transporter that functions to control transporter substrate preference during plant growth and defense balance decisions.

Coronatine promotes maize water uptake by directly binding to the aquaporin ZmPIP2;5 and enhancing its activity

Water uptake is crucial for crop growth and development and drought stress tolerance. The water channel aquaporins (AQP) play important roles in plant water uptake. Here, we discovered that a jasmonic acid analog, coronatine (COR), enhanced maize (Zea mays) root water uptake capacity under artificial water deficiency conditions. COR treatment induced the expression of the AQP gene Plasma membrane intrinsic protein 2;5 (ZmPIP2;5). In vivo and in vitro experiments indicated that COR also directly acts on ZmPIP2;5 to improve water uptake in maize and Xenopus oocytes. The leaf water potential and hydraulic conductivity of roots growing under hyperosmotic conditions were higher in ZmPIP2;5-overexpression lines and lower in the zmpip2;5 knockout mutant, compared to wild-type plants. Based on a comparison between ZmPIP2;5 and other PIP2s, we predicted that COR may bind to the functional site in loop E of ZmPIP2;5. We confirmed this prediction by surface plasmon resonance technology and a microscale thermophoresis assay, and showed that deleting the binding motif greatly reduced COR binding. We identified the N241 residue as the COR-specific binding site, which may activate the channel of the AQP tetramer and increase water transport activity, which may facilitate water uptake under hyperosmotic stress.

Regulation of PIN-FORMED Protein Degradation

Auxin action largely depends on the establishment of auxin concentration gradient within plant organs, where PIN-formed (PIN) auxin transporter-mediated directional auxin movement plays an important role. Accumulating studies have revealed the need of polar plasma membrane (PM) localization of PIN proteins as well as regulation of PIN polarity in response to developmental cues and environmental stimuli, amongst which a typical example is regulation of PIN phosphorylation by AGCVIII protein kinases and type A regulatory subunits of PP2A phosphatases. Recent findings, however, highlight the importance of PIN degradation in reestablishing auxin gradient. Although the underlying mechanism is poorly understood, these findings provide a novel aspect to broaden the current knowledge on regulation of polar auxin transport. In this review, we summarize the current understanding on controlling PIN degradation by endosome-mediated vacuolar targeting, autophagy, ubiquitin modification and the related E3 ubiquitin ligases, cytoskeletons, plant hormones, environmental stimuli, and other regulators, and discuss the possible mechanisms according to recent studies.

Functions of the Hsp90-Binding FKBP Immunophilins

The Hsp90 chaperone is known to interact with a diverse array of client proteins. However, in every case examined, Hsp90 is also accompanied by a single or several co-chaperone proteins. One class of co-chaperone contains a tetratricopeptide repeat (TPR) domain that targets the co-chaperone to the C-terminal region of Hsp90. Within this class are Hsp90-binding peptidylprolyl isomerases, most of which belong to the FK506-binding protein (FKBP) family. Despite the common association of FKBP co-chaperones with Hsp90, it is abundantly clear that the client protein influences, and is often influenced by, the particular FKBP bound to Hsp90. Examples include Xap2 in aryl hydrocarbon receptor complexes and FKBP52 in steroid receptor complexes. In this chapter, we discuss the known functional roles played by FKBP co-chaperones and, where possible, relate distinctive functions to structural differences between FKBP members.

Actin isovariant ACT7 controls root meristem development in Arabidopsis through modulating auxin and ethylene responses

The meristem is the most functionally dynamic part in a plant. The shaping of the meristem requires constant cell division and elongation, which are influenced by hormones and the cytoskeletal component, actin. Although the roles of hormones in modulating meristem development have been extensively studied, the role of actin in this process is still elusive. Using the single and double mutants of the vegetative class actin, we demonstrate that actin isovariant ACT7 plays an important role in root meristem development. In the absence of ACT7, but not ACT8 and ACT2, depolymerization of actin was observed. Consistently, the act7 mutant showed reduced cell division, cell elongation, and meristem length. Intracellular distribution and trafficking of auxin transport proteins in the actin mutants revealed that ACT7 specifically functions in the root meristem to facilitate the trafficking of auxin efflux carriers PIN1 and PIN2, and consequently the transport of auxin. Compared with act7, the act7act8 double mutant exhibited slightly enhanced phenotypic response and altered intracellular trafficking. The altered distribution of auxin in act7 and act7act8 affects the response of the roots to ethylene, but not to cytokinin. Collectively, our results suggest that ACT7-dependent auxin-ethylene response plays a key role in controlling Arabidopsis root meristem development.

The unconventional prefoldin RPB5 interactor mediates the gravitropic response by modulating cytoskeleton organization and auxin transport in Arabidopsis

Gravity-induced root curvature involves the asymmetric distribution of the phytohormone auxin. This response depends on the concerted activities of the auxin transporters such as PIN-FORMED (PIN) proteins for auxin efflux and AUXIN RESISTANT 1 (AUX1) for auxin influx. However, how the auxin gradient is established remains elusive. Here we identified a new mutant with a short root, strong auxin distribution in the lateral root cap and an impaired gravitropic response. The causal gene encoded an Arabidopsis homolog of the human unconventional prefoldin RPB5 interactor (URI). AtURI interacted with prefoldin 2 (PFD2) and PFD6, two β-type PFD members that modulate actin and tubulin patterning in roots. The auxin reporter DR5_rev :GFP showed that asymmetric auxin redistribution after gravistimulation is disordered in aturi-1 root tips. Treatment with the endomembrane protein trafficking inhibitor brefeldin A indicated that recycling of the auxin transporter PIN2 is disrupted in aturi-1 roots as well as in pfd mutants. We propose that AtURI cooperates with PFDs to recycle PIN2 and modulate auxin distribution.

Arabidopsis TWISTED DWARF1 regulates stamen elongation by differential activation of ABCB1,19-mediated auxin transport

Despite clear evidence that a local accumulation of auxin is likewise critical for male fertility, much less is known about the components that regulate auxin-controlled stamen development. In this study, we analyzed physiological and morphological parameters in mutants of key players of ABCB-mediated auxin transport, and spatially and temporally dissected their expression on the protein level as well as auxin fluxes in the Arabidopsis stamens. Our analyses revealed that the FKBP42, TWISTED DWARF1 (TWD1), promotes stamen elongation and, to a lesser extent, anther dehiscence, as well as pollen maturation, and thus is required for seed development. Most of the described developmental defects in twd1 are shared with the abcb1 abcb19 mutant, which can be attributed to the fact that TWD1-as a described ABCB chaperone-is a positive regulator of ABCB1- and ABCB19-mediated auxin transport. However, reduced stamen number was dependent on TWD1 but not on investigated ABCBs, suggesting additional players downstream of TWD1. We predict an overall housekeeping function for ABCB1 during earlier stages, while ABCB19 seems to be responsible for the key event of rapid elongation at later stages of stamen development. Our data indicate that TWD1 controls stamen development by differential activation of ABCB1,19-mediated auxin transport in the stamen.

Structures and mechanisms of the Arabidopsis auxin transporter PIN3

The PIN-FORMED (PIN) protein family of auxin transporters mediates the polar auxin transport and plays crucial roles in plant growth and development^1,2. Here we present cryo-EM structures of PIN3 from Arabidopsis thaliana (AtPIN3) in the apo state and in complex with its substrate indole-3-acetic acid (IAA) and the inhibitor N-1-naphthylphthalamic acid (NPA) at 2.6-3.0 Å resolution. AtPIN3 exists as a homodimer, with the transmembrane helices (TMs) 1, 2, and 7 in the scaffold domain involved in dimerization. The dimeric AtPIN3 forms a large, joint extracellular-facing cavity at the dimer interface while each subunit adopts an inward-facing conformation. The structural and functional analyses, along with computational studies, reveal the structural basis for the recognition of IAA and NPA and elucidate the molecular mechanism of NPA inhibition on the PIN-mediated auxin transport. The AtPIN3 structures support an elevator-like model for the transport of auxin, whereby the transport domains undergo up-down rigid-body motions and the dimerized scaffold domains remain static.

Fourteen Stations of Auxin

Auxin has always been at the forefront of research in plant physiology and development. Since the earliest contemplations by Julius von Sachs and Charles Darwin, more than a century-long struggle has been waged to understand its function. This largely reflects the failures, successes, and inevitable progress in the entire field of plant signaling and development. Here I present 14 stations on our long and sometimes mystical journey to understand auxin. These highlights were selected to give a flavor of the field and to show the scope and limits of our current knowledge. A special focus is put on features that make auxin unique among phytohormones, such as its dynamic, directional transport network, which integrates external and internal signals, including self-organizing feedback. Accented are persistent mysteries and controversies. The unexpected discoveries related to rapid auxin responses and growth regulation recently disturbed our contentment regarding understanding of the auxin signaling mechanism. These new revelations, along with advances in technology, usher us into a new, exciting era in auxin research.

Loss of Multiple ABCB Auxin Transporters Recapitulates the Major twisted dwarf 1 Phenotypes in Arabidopsis thaliana

FK506-BINDING PROTEIN 42/TWISTED DWARF 1 (FKBP42/TWD1) directly regulates cellular trafficking and activation of multiple ATP-BINDING CASSETTE (ABC) transporters from the ABCB and ABCC subfamilies. abcb1 abcb19 double mutants exhibit remarkable phenotypic overlap with twd1 including severe dwarfism, stamen elongation defects, and compact circinate leaves; however, twd1 mutants exhibit greater loss of polar auxin transport and additional helical twisting of roots, inflorescences, and siliques. As abcc1 abcc2 mutants do not exhibit any visible phenotypes and TWD1 does not interact with PIN or AUX1/LAX auxin transporters, loss of function of other ABCB auxin transporters is hypothesized to underly the remaining morphological phenotypes. Here, gene expression, mutant analyses, pharmacological inhibitor studies, auxin transport assays, and direct auxin quantitations were used to determine the relative contributions of loss of other reported ABCB auxin transporters (4, 6, 11, 14, 20, and 21) to twd1 phenotypes. From these analyses, the additional reduction in plant height and the twisted inflorescence, root, and silique phenotypes observed in twd1 compared to abcb1 abcb19 result from loss of ABCB6 and ABCB20 function. Additionally, abcb6 abcb20 root twisting exhibited the same sensitivity to the auxin transport inhibitor 1-napthalthalamic acid as twd1 suggesting they are the primary contributors to these auxin-dependent organ twisting phenotypes. The lack of obvious phenotypes in higher order abcb4 and abcb21 mutants suggests that the functional loss of these transporters does not contribute to twd1 root or shoot twisting. Analyses of ABCB11 and ABCB14 function revealed capacity for auxin transport; however, their activities are readily outcompeted by other substrates, suggesting alternate functions in planta, consistent with a spectrum of relative substrate affinities among ABCB transporters. Overall, the results presented here suggest that the ABCB1/19 and ABCB6/20 pairs represent the primary long-distance ABCB auxin transporters in Arabidopsis and account for all reported twd1 morphological phenotypes. Other ABCB transporters appear to participate in highly localized auxin streams or mobilize alternate transport substrates.

HSP90 affects root growth in Arabidopsis by regulating the polar distribution of PIN1

Auxin homeostasis and signaling affect a broad range of developmental processes in plants. The interplay between HSP90 and auxin signaling is channeled through the chaperoning capacity of the HSP90 on the TIR1 auxin receptor. The sophisticated buffering capacity of the HSP90 system through the interaction with diverse signaling protein components drastically shapes genetic circuitries regulating various developmental aspects. However, the elegant networking capacity of HSP90 in the global regulation of auxin response and homeostasis has not been appreciated. Arabidopsis hsp90 mutants were screened for gravity response. Phenotypic analysis of root meristems and cotyledon veins was performed. PIN1 localization in hsp90 mutants was determined. Our results showed that HSP90 affected the asymmetrical distribution of PIN1 in plasma membranes and influenced its expression in prompt cell niches. Depletion of HSP90 distorted polar distribution of auxin, as the acropetal auxin transport was highly affected, leading to impaired root gravitropism and lateral root formation. The essential role of the HSP90 in auxin homeostasis was profoundly evident from early development, as HSP90 depletion affected embryo development and the pattern formation of veins in cotyledons. Our data suggest that the HSP90-mediated distribution of PIN1 modulates auxin distribution and thereby auxin signaling to properly promote plant development.

Chemical Biology in Auxin Research

Molecular genetic and structural studies have revealed the mechanisms of fundamental components of key auxin regulatory pathways consisting of auxin biosynthesis, transport, and signaling. Chemical biology methods applied in auxin research have been greatly expanded through the understanding of auxin regulatory pathways. Many small-molecule modulators of auxin metabolism, transport, and signaling have been generated on the basis of the outcomes of genetic and structural studies on auxin regulatory pathways. These chemical modulators are now widely used as essential tools for dissecting auxin biology in diverse plants. This review covers the structures, primary targets, modes of action, and applications of chemical tools in auxin biosynthesis, transport, and signaling.

Cell kinetics of auxin transport and activity in Arabidopsis root growth and skewing

Auxin is a key regulator of plant growth and development. Local auxin biosynthesis and intercellular transport generates regional gradients in the root that are instructive for processes such as specification of developmental zones that maintain root growth and tropic responses. Here we present a toolbox to study auxin-mediated root development that features: (i) the ability to control auxin synthesis with high spatio-temporal resolution and (ii) single-cell nucleus tracking and morphokinetic analysis infrastructure. Integration of these two features enables cutting-edge analysis of root development at single-cell resolution based on morphokinetic parameters under normal growth conditions and during cell-type-specific induction of auxin biosynthesis. We show directional auxin flow in the root and refine the contributions of key players in this process. In addition, we determine the quantitative kinetics of Arabidopsis root meristem skewing, which depends on local auxin gradients but does not require PIN2 and AUX1 auxin transporter activities. Beyond the mechanistic insights into root development, the tools developed here will enable biologists to study kinetics and morphology of various critical processes at the single cell-level in whole organisms.

Arabidopsis QWRF1 and QWRF2 Redundantly Modulate Cortical Microtubule Arrangement in Floral Organ Growth and Fertility

Floral organ development is fundamental to sexual reproduction in angiosperms. Many key floral regulators (most of which are transcription factors) have been identified and shown to modulate floral meristem determinacy and floral organ identity, but not much is known about the regulation of floral organ growth, which is a critical process by which organs to achieve appropriate morphologies and fulfill their functions. Spatial and temporal control of anisotropic cell expansion following initial cell proliferation is important for organ growth. Cortical microtubules are well known to have important roles in plant cell polar growth/expansion and have been reported to guide the growth and shape of sepals and petals. In this study, we identified two homolog proteins, QWRF1 and QWRF2, which are essential for floral organ growth and plant fertility. We found severely deformed morphologies and symmetries of various floral organs as well as a significant reduction in the seed setting rate in the qwrf1qwrf2 double mutant, although few flower development defects were seen in qwrf1 or qwrf2 single mutants. QWRF1 and QWRF2 display similar expression patterns and are both localized to microtubules in vitro and in vivo. Furthermore, we found altered cortical microtubule organization and arrangements in qwrf1qwrf2 cells, consistent with abnormal cell expansion in different floral organs, which eventually led to poor fertility. Our results suggest that QWRF1 and QWRF2 are likely microtubule-associated proteins with functional redundancy in fertility and floral organ development, which probably exert their effects via regulation of cortical microtubules and anisotropic cell expansion.

Naphthylphthalamic acid associates with and inhibits PIN auxin transporters

N-1-naphthylphthalamic acid (NPA) is a key inhibitor of directional (polar) transport of the hormone auxin in plants. For decades, it has been a pivotal tool in elucidating the unique polar auxin transport-based processes underlying plant growth and development. Its exact mode of action has long been sought after and is still being debated, with prevailing mechanistic schemes describing only indirect connections between NPA and the main transporters responsible for directional transport, namely PIN auxin exporters. Here we present data supporting a model in which NPA associates with PINs in a more direct manner than hitherto postulated. We show that NPA inhibits PIN activity in a heterologous oocyte system and that expression of NPA-sensitive PINs in plant, yeast, and oocyte membranes leads to specific saturable NPA binding. We thus propose that PINs are a bona fide NPA target. This offers a straightforward molecular basis for NPA inhibition of PIN-dependent auxin transport and a logical parsimonious explanation for the known physiological effects of NPA on plant growth, as well as an alternative hypothesis to interpret past and future results. We also introduce PIN dimerization and describe an effect of NPA on this, suggesting that NPA binding could be exploited to gain insights into structural aspects of PINs related to their transport mechanism.

Pho-view of Auxin: Reversible Protein Phosphorylation in Auxin Biosynthesis, Transport and Signaling

The phytohormone auxin plays a central role in shaping plant growth and development. With decades of genetic and biochemical studies, numerous core molecular components and their networks, underlying auxin biosynthesis, transport, and signaling, have been identified. Notably, protein phosphorylation, catalyzed by kinases and oppositely hydrolyzed by phosphatases, has been emerging to be a crucial type of post-translational modification, regulating physiological and developmental auxin output at all levels. In this review, we comprehensively discuss earlier and recent advances in our understanding of genetics, biochemistry, and cell biology of the kinases and phosphatases participating in auxin action. We provide insights into the mechanisms by which reversible protein phosphorylation defines developmental auxin responses, discuss current challenges, and provide our perspectives on future directions involving the integration of the control of protein phosphorylation into the molecular auxin network.

Flavonol-mediated stabilization of PIN efflux complexes regulates polar auxin transport

The transport of auxin controls the rate, direction and localization of plant growth and development. The course of auxin transport is defined by the polar subcellular localization of the PIN proteins, a family of auxin efflux transporters. However, little is known about the composition and regulation of the PIN protein complex. Here, using blue-native PAGE and quantitative mass spectrometry, we identify native PIN core transport units as homo- and heteromers assembled from PIN1, PIN2, PIN3, PIN4 and PIN7 subunits only. Furthermore, we show that endogenous flavonols stabilize PIN dimers to regulate auxin efflux in the same way as does the auxin transport inhibitor 1-naphthylphthalamic acid (NPA). This inhibitory mechanism is counteracted both by the natural auxin indole-3-acetic acid and by phosphomimetic amino acids introduced into the PIN1 cytoplasmic domain. Our results lend mechanistic insights into an endogenous control mechanism which regulates PIN function and opens the way for a deeper understanding of the protein environment and regulation of the polar auxin transport complex.

Investigation of Auxin Biosynthesis and Action Using Auxin Biosynthesis Inhibitors

Auxin plays important roles in almost all aspects of plant growth and development. Chemical genetics is an effective approach to understand auxin action, especially in nonmodel plant species, in which auxin-related mutants are not yet available. Among auxin-related chemical tools, we present approaches to utilize auxin biosynthesis inhibitors. The inhibitors are effective not only to understand auxin biosynthesis but also to understand auxin action. The effectiveness of the inhibitors can be assessed based on in vitro or in vivo assays. The in vitro assay employs enzyme inhibition assays. The in vivo assay employs UPLC-MS/MS-based analysis of endogenous IAA and its intermediates or metabolites.

Bundling up the Role of the Actin Cytoskeleton in Primary Root Growth

Primary root growth is required by the plant to anchor in the soil and reach out for nutrients and water, while dealing with obstacles. Efficient root elongation and bending depends upon the coordinated action of environmental sensing, signal transduction, and growth responses. The actin cytoskeleton is a highly plastic network that constitutes a point of integration for environmental stimuli and hormonal pathways. In this review, we present a detailed compilation highlighting the importance of the actin cytoskeleton during primary root growth and we describe how actin-binding proteins, plant hormones, and actin-disrupting drugs affect root growth and root actin. We also discuss the feedback loop between actin and root responses to light and gravity. Actin affects cell division and elongation through the control of its own organization. We remark upon the importance of longitudinally oriented actin bundles as a hallmark of cell elongation as well as the role of the actin cytoskeleton in protein trafficking and vacuolar reshaping during this process. The actin network is shaped by a plethora of actin-binding proteins; however, there is still a large gap in connecting the molecular function of these proteins with their developmental effects. Here, we summarize their function and known effects on primary root growth with a focus on their high level of specialization. Light and gravity are key factors that help us understand root growth directionality. The response of the root to gravity relies on hormonal, particularly auxin, homeostasis, and the actin cytoskeleton. Actin is necessary for the perception of the gravity stimulus via the repositioning of sedimenting statoliths, but it is also involved in mediating the growth response via the trafficking of auxin transporters and cell elongation. Furthermore, auxin and auxin analogs can affect the composition of the actin network, indicating a potential feedback loop. Light, in its turn, affects actin organization and hence, root growth, although its precise role remains largely unknown. Recently, fundamental studies with the latest techniques have given us more in-depth knowledge of the role and organization of actin in the coordination of root growth; however, there remains a lot to discover, especially in how actin organization helps cell shaping, and therefore root growth.

Non-steroidal Anti-inflammatory Drugs Target TWISTED DWARF1-Regulated Actin Dynamics and Auxin Transport-Mediated Plant Development

The widely used non-steroidal anti-inflammatory drugs (NSAIDs) are derivatives of the phytohormone salicylic acid (SA). SA is well known to regulate plant immunity and development, whereas there have been few reports focusing on the effects of NSAIDs in plants. Our studies here reveal that NSAIDs exhibit largely overlapping physiological activities to SA in the model plant Arabidopsis. NSAID treatments lead to shorter and agravitropic primary roots and inhibited lateral root organogenesis. Notably, in addition to the SA-like action, which in roots involves binding to the protein phosphatase 2A (PP2A), NSAIDs also exhibit PP2A-independent effects. Cell biological and biochemical analyses reveal that many NSAIDs bind directly to and inhibit the chaperone activity of TWISTED DWARF1, thereby regulating actin cytoskeleton dynamics and subsequent endosomal trafficking. Our findings uncover an unexpected bioactivity of human pharmaceuticals in plants and provide insights into the molecular mechanism underlying the cellular action of this class of anti-inflammatory compounds.

A twist in the ABC: regulation of ABC transporter trafficking and transport by FK506-binding proteins

Post-transcriptional regulation of ATP-binding cassette (ABC) proteins has been so far shown to encompass protein phosphorylation, maturation, and ubiquitination. Yet, recent accumulating evidence implicates FK506-binding proteins (FKBPs), a type of peptidylprolyl cis-trans isomerase (PPIase) proteins, in ABC transporter regulation. In this perspective article, we summarize current knowledge on ABC transporter regulation by FKBPs, which seems to be conserved over kingdoms and ABC subfamilies. We uncover striking functional similarities but also differences between regulatory FKBP-ABC modules in plants and mammals. We dissect a PPIase- and HSP90-dependent and independent impact of FKBPs on ABC biogenesis and transport activity. We propose and discuss a putative new mode of transient ABC transporter regulation by cis-trans isomerization of X-prolyl bonds.

Auxin-transporting ABC transporters are defined by a conserved D/E-P motif regulated by a prolylisomerase

The plant hormone auxin must be transported throughout plants in a cell-to-cell manner to affect its various physiological functions. ABCB transporters are critical for this polar auxin distribution, but the regulatory mechanisms controlling their function is not fully understood. The auxin transport activity of ABCB1 was suggested to be regulated by a physical interaction with FKBP42/Twisted Dwarf1 (TWD1), a peptidylprolyl cis-trans isomerase (PPIase), but all attempts to demonstrate such a PPIase activity by TWD1 have failed so far. By using a structure-based approach, we identified several surface-exposed proline residues in the nucleotide binding domain and linker of Arabidopsis ABCB1, mutations of which do not alter ABCB1 protein stability or location but do affect its transport activity. P1008 is part of a conserved signature D/E-P motif that seems to be specific for auxin-transporting ABCBs, which we now refer to as ATAs. Mutation of the acidic residue also abolishes auxin transport activity by ABCB1. All higher plant ABCBs for which auxin transport has been conclusively proven carry this conserved motif, underlining its predictive potential. Introduction of this D/E-P motif into malate importer, ABCB14, increases both its malate and its background auxin transport activity, suggesting that this motif has an impact on transport capacity. The D/E-P1008 motif is also important for ABCB1-TWD1 interactions and activation of ABCB1-mediated auxin transport by TWD1. In summary, our data imply a new function for TWD1 acting as a putative activator of ABCB-mediated auxin transport by cis-trans isomerization of peptidyl-prolyl bonds.

Root-Apex Proton Fluxes at the Centre of Soil-Stress Acclimation

Proton (H⁺) fluxes in plant roots play critical roles in maintaining root growth and facilitating plant responses to multiple soil stresses, including fluctuations in nutrient supply, salt infiltration, and water stress. Soil mining for nutrients and water, rates of nutrient uptake, and the modulation of cell expansion all depend on the regulation of root H⁺ fluxes, particularly at the root apex, mediated primarily by the activity of plasma membrane (PM) H⁺-ATPases. Here, we summarize recent findings on the regulatory mechanisms of H⁺ fluxes at the root apex under three abiotic stress conditions - phosphate deficiency, salinity stress, and water deficiency - and present an integrated physiomolecular view of the functions of H⁺ fluxes in maintaining root growth in the acclimation to soil stress.

Multifaceted roles of HEAT SHOCK PROTEIN 90 molecular chaperones in plant development

HEAT SHOCK PROTEINS 90 (HSP90s) are molecular chaperones that mediate correct folding and stability of many client proteins. These chaperones act as master molecular hubs involved in multiple aspects of cellular and developmental signalling in diverse organisms. Moreover, environmental and genetic perturbations affect both HSP90s and their clients, leading to alterations of molecular networks determining respectively plant phenotypes and genotypes and contributing to a broad phenotypic plasticity. Although HSP90 interaction networks affecting the genetic basis of phenotypic variation and diversity have been thoroughly studied in animals, such studies are just starting to emerge in plants. Here, we summarize current knowledge and discuss HSP90 network functions in plant development and cellular homeostasis.

Regulation of Three Key Kinases of Brassinosteroid Signaling Pathway

Brassinosteroids (BRs) are important plant growth hormones that regulate a wide range of plant growth and developmental processes. The BR signals are perceived by two cell surface-localized receptor kinases, Brassinosteroid-Insensitive1 (BRI1) and BRI1-Associated receptor Kinase (BAK1), and reach the nucleus through two master transcription factors, bri1-EMS suppressor1 (BES1) and Brassinazole-resistant1 (BZR1). The intracellular transmission of the BR signals from BRI1/BAK1 to BES1/BZR1 is inhibited by a constitutively active kinase Brassinosteroid-Insensitive2 (BIN2) that phosphorylates and negatively regulates BES1/BZR1. Since their initial discoveries, further studies have revealed a plethora of biochemical and cellular mechanisms that regulate their protein abundance, subcellular localizations, and signaling activities. In this review, we provide a critical analysis of the current literature concerning activation, inactivation, and other regulatory mechanisms of three key kinases of the BR signaling cascade, BRI1, BAK1, and BIN2, and discuss some unresolved controversies and outstanding questions that require further investigation.

Wounding-induced changes in cellular pressure and localized auxin signalling spatially coordinate restorative divisions in roots

Plants are sessile organisms that cannot evade wounding or pathogen attack, and their cells are encapsulated within cell walls, making it impossible to use cell migration for wound healing like animals. Thus, regeneration in plants largely relies on the coordination of targeted cell expansion and oriented cell division. Here we show in the root that the major growth hormone auxin is specifically activated in wound-adjacent cells, regulating cell expansion, cell division rates, and regeneration-involved transcription factor ERF115. These wound responses depend on cell collapse of the eliminated cells presumably perceived by the cell damage-induced changes in cellular pressure. This largely broadens our understanding of how wound responses are coordinated on a cellular level to mediate wound healing and prevent overproliferation.

Wound healing in plant tissues, consisting of rigid cell wall-encapsulated cells, represents a considerable challenge and occurs through largely unknown mechanisms distinct from those in animals. Owing to their inability to migrate, plant cells rely on targeted cell division and expansion to regenerate wounds. Strict coordination of these wound-induced responses is essential to ensure efficient, spatially restricted wound healing. Single-cell tracking by live imaging allowed us to gain mechanistic insight into the wound perception and coordination of wound responses after laser-based wounding in Arabidopsis root. We revealed a crucial contribution of the collapse of damaged cells in wound perception and detected an auxin increase specific to cells immediately adjacent to the wound. This localized auxin increase balances wound-induced cell expansion and restorative division rates in a dose-dependent manner, leading to tumorous overproliferation when the canonical TIR1 auxin signaling is disrupted. Auxin and wound-induced turgor pressure changes together also spatially define the activation of key components of regeneration, such as the transcription regulator ERF115. Our observations suggest that the wound signaling involves the sensing of collapse of damaged cells and a local auxin signaling activation to coordinate the downstream transcriptional responses in the immediate wound vicinity.

Auxin-induced actin cytoskeleton rearrangements require AUX1

The actin cytoskeleton is required for cell expansion and implicated in cellular responses to the phytohormone auxin. However, the mechanisms that coordinate auxin signaling, cytoskeletal remodeling and cell expansion are poorly understood. Previous studies examined long-term actin cytoskeleton responses to auxin, but plants respond to auxin within minutes. Before this work, an extracellular auxin receptor - rather than the auxin transporter AUXIN RESISTANT 1 (AUX1) - was considered to precede auxin-induced cytoskeleton reorganization. In order to correlate actin array organization and dynamics with degree of cell expansion, quantitative imaging tools established baseline actin organization and illuminated individual filament behaviors in root epidermal cells under control conditions and after indole-3-acetic acid (IAA) application. We evaluated aux1 mutant actin organization responses to IAA and the membrane-permeable auxin 1-naphthylacetic acid (NAA). Cell length predicted actin organization and dynamics in control roots; short-term IAA treatments stimulated denser and more parallel, longitudinal arrays by inducing filament unbundling within minutes. Although AUX1 is necessary for full actin rearrangements in response to auxin, cytoplasmic auxin (i.e. NAA) stimulated a lesser response. Actin filaments became more 'organized' after IAA stopped elongation, refuting the hypothesis that 'more organized' actin arrays universally correlate with rapid growth. Short-term actin cytoskeleton response to auxin requires AUX1 and/or cytoplasmic auxin.

Rapid Detection of Hormonal Involvement in Light Responses

Many aspects of light-controlled metabolism and development of plants depend on hormonal pathways. Here, a method is described to identify such hormonal dependence in light-regulated processes. A number of compounds-hormones and chemicals which interfere with hormonal pathways-are listed because of their usefulness in pharmacological treatment experiments. As an example for practical use of such compounds, elongation growth is discussed. An experimental setup is described in which plants are grown so that their structures develop predominantly in a two-dimensional plane. Time-lapse imaging is used to follow the plants in time, and image analysis reveals changes in plant morphology.

Proteomic analysis of a clavata-like phenotype mutant in Brassica napus

Rapeseed is one of important oil crops in China. Better understanding of the regulation network of main agronomic traits of rapeseed could improve the yielding of rapeseed. In this study, we obtained an influrescence mutant that showed a fusion phenotype, similar with the Arabidopsis clavata-like phenotype, so we named the mutant as Bnclavata-like (Bnclv-like). Phenotype analysis illustrated that abnormal development of the inflorescence meristem (IM) led to the fused-inflorescence phenotype. At the stage of protein abundance, major regulators in metabolic processes, ROS metabolism, and cytoskeleton formation were seen to be altered in this mutant. These results not only revealed the relationship between biological processes and inflorescence meristem development, but also suggest bioengineering strategies for the improved breeding and production of Brassica napus.

Endogenous auxin determines the pattern of adventitious shoot formation on internodal segments of ipecac

Endogenous auxin determines the pattern of adventitious shoot formation. Auxin produced in the dominant shoot is transported to the internodal segment and suppresses growth of other shoots. Adventitious shoot formation is required for the propagation of economically important crops and for the regeneration of transgenic plants. In most plant species, phytohormones are added to culture medium to induce adventitious shoots. In ipecac (Carapichea ipecacuanha (Brot.) L. Andersson), however, adventitious shoots can be formed without phytohormone treatment. Thus, ipecac culture allows us to investigate the effects of endogenous phytohormones during adventitious shoot formation. In phytohormone-free culture, adventitious shoots were formed on the apical region of the internodal segments, and a high concentration of IAA was detected in the basal region. To explore the relationship between endogenous auxin and adventitious shoot formation, we evaluated the effects of auxin transport inhibitors, auxin antagonists, and auxin biosynthesis inhibitors on adventitious shoot formation in ipecac. Auxin antagonists and biosynthesis inhibitors strongly suppressed adventitious shoot formation, which was restored by exogenously applied auxin. Auxin biosynthesis and transport inhibitors significantly decreased the IAA level in the basal region and shifted the positions of adventitious shoot formation from the apical region to the middle region of the segments. These data indicate that auxin determines the positions of the shoots formed on internodal segments of ipecac. Only one of the shoots formed grew vigorously; this phenomenon is similar to apical dominance. When the largest shoot was cut off, other shoots started to grow. Naphthalene-1-acetic acid treatment of the cut surface suppressed shoot growth, indicating that auxin produced in the dominant shoot is transported to the internodal segment and suppresses growth of other shoots.

Is naphthylphthalamic acid a specific phytotropin? It elevates ethylene and alters metabolic homeostasis in tomato

In higher plants, phytohormone indole-3-acetic acid is characteristically transported from the apex towards the base of the plant, termed as polar auxin transport (PAT). Among the inhibitors blocking PAT, N-1-naphthylphthalamic acid (NPA) that targets ABCB transporters is most commonly used. NPA-treated light-grown Arabidopsis seedlings show severe inhibition of hypocotyl and root elongation. In light-grown tomato seedlings, NPA inhibited root growth, but contrary to Arabidopsis stimulated hypocotyl elongation. The NPA-stimulation of hypocotyl elongation was milder in blue, red, and far-red light-grown seedlings. The NPA-treatment stimulated emission of ethylene from the seedlings. The scrubbing of ethylene by mercuric perchlorate reduced NPA-stimulated hypocotyl elongation. NPA action on hypocotyl elongation was antagonized by 1-methylcyclopropene, an inhibitor of ethylene action. NPA-treated seedlings had reduced levels of indole-3-butyric acid and higher levels of zeatin in the shoots. NPA did not alter indole-3-acetic levels in shoots. The analysis of metabolic networks indicated that NPA-treatment induced moderate shifts in the networks compared to exogenous ethylene that induced a drastic shift in metabolic networks. Our results indicate that in addition to ethylene, NPA-stimulated hypocotyl elongation in tomato may also involve zeatin and indole-3- butyric acid. Our results indicate that NPA-mediated physiological responses may vary in a species-specific fashion.

An Auxin Transport Inhibitor Targets Villin-Mediated Actin Dynamics to Regulate Polar Auxin Transport

Auxin transport inhibitors are essential tools for understanding auxin-dependent plant development. One mode of inhibition affects actin dynamics; however, the underlying mechanisms remain unclear. In this study, we characterized the action of 2,3,5-triiodobenzoic acid (TIBA) on actin dynamics in greater mechanistic detail. By surveying mutants for candidate actin-binding proteins with reduced TIBA sensitivity, we determined that Arabidopsis (Arabidopsis thaliana) villins contribute to TIBA action. By directly interacting with the C-terminal headpiece domain of villins, TIBA causes villin to oligomerize, driving excessive bundling of actin filaments. The resulting changes in actin dynamics impair auxin transport by disrupting the trafficking of PIN-FORMED auxin efflux carriers and reducing their levels at the plasma membrane. Collectively, our study provides mechanistic insight into the link between the actin cytoskeleton, vesicle trafficking, and auxin transport.

Inclusion Complexes of β and HPβ-Cyclodextrin with α, β Amyrin and In Vitro Anti-Inflammatory Activity

α, β amyrin (ABAM) is a natural mixture of pentacyclic triterpenes that has a wide range of biological activities. ABAM is isolated from the species of the Burseraceae family, in which the species Protium is commonly found in the Amazon region of Brazil. The aim of this work was to develop inclusion complexes (ICs) of ABAM and β-cyclodextrin (βCD) and hydroxypropyl-β-cyclodextrin (HPβCD) by physical mixing (PM) and kneading (KN) methods. Interactions between ABAM and the CD’s as well as the formation of ICs were confirmed by physicochemical characterization in the solid state by Fourier transform infrared (FTIR), scanning electron microscopy (SEM), X-ray diffraction (XRD), thermogravimetry (TG) and differential scanning calorimetry (DSC). Physicochemical characterization indicated the formation of ICs with both βCD and HPβCD. Such ICs were able to induce changes in the physicochemical properties of ABAM. In addition, the formation of ICs with cyclodextrins showed to be an effective and promising alternative to enhance the anti-inflammatory activity and safety of ABAM.

A Mobile Auxin Signal Connects Temperature Sensing in Cotyledons with Growth Responses in Hypocotyls

Plants have a remarkable capacity to adjust their growth and development to elevated ambient temperatures. Increased elongation growth of roots, hypocotyls, and petioles in warm temperatures are hallmarks of seedling thermomorphogenesis. In the last decade, significant progress has been made to identify the molecular signaling components regulating these growth responses. Increased ambient temperature utilizes diverse components of the light sensing and signal transduction network to trigger growth adjustments. However, it remains unknown whether temperature sensing and responses are universal processes that occur uniformly in all plant organs. Alternatively, temperature sensing may be confined to specific tissues or organs, which would require a systemic signal that mediates responses in distal parts of the plant. Here, we show that Arabidopsis (Arabidopsis thaliana) seedlings show organ-specific transcriptome responses to elevated temperatures and that thermomorphogenesis involves both autonomous and organ-interdependent temperature sensing and signaling. Seedling roots can sense and respond to temperature in a shoot-independent manner, whereas shoot temperature responses require both local and systemic processes. The induction of cell elongation in hypocotyls requires temperature sensing in cotyledons, followed by the generation of a mobile auxin signal. Subsequently, auxin travels to the hypocotyl, where it triggers local brassinosteroid-induced cell elongation in seedling stems, which depends upon a distinct, permissive temperature sensor in the hypocotyl.

Spatial Distribution of Biomaterial Microenvironment pH and Its Modulatory Effect on Osteoclasts at the Early Stage of Bone Defect Regeneration

It is generally accepted that biodegradable materials greatly influence the nearby microenvironment where cells reside; however, the range of interfacial properties has seldom been discussed due to technical bottlenecks. This study aims to depict biomaterial microenvironment boundaries by correlating interfacial H⁺ distribution with surrounding cell behaviors. Using a disuse-related osteoporotic mouse model, we confirmed that the abnormal activated osteoclasts could be suppressed under relatively alkaline conditions. The differentiation and apatite-resorption capability of osteoclasts were "switched off" when cultured in titrated material extracts with pH values higher than 7.8. To generate a localized alkaline microenvironment, a series of borosilicates were fabricated and their interfacial H⁺ distributions were monitored spatiotemporally by employing noninvasive microtest technology. By correlating interfacial H⁺ distribution with osteoclast "switch on/off" behavior, the microenvironment boundary of the tested material was found to be 400 ± 50 μm, which is broader than the generally accepted value, 300 μm. Furthermore, osteoporotic mice implanted with materials with higher interfacial pH values and boarder effective ranges had lower osteoclast activities and a thicker new bone. To conclude, effective proton microenvironment boundaries of degradable biomaterials were depicted and a weak alkaline microenvironment was shown to promote regeneration of osteoporotic bones possibly by suppressing abnormal activated osteoclasts.